Pulse translator



March 1, 1949. D. D. GRIEG 2,462,860

PULSE TRANSLATOR i Filed March 19, 1945 2 Sheets-Sheet 1 A TTRNEY Marchl, 1949.

Filed March 19, 1945 D. D. GRIEG 2,462,860

PULSE TRANSLATOR 2 SheetsSheet 2 Pulsar/war Patented Mar. 1, 1949 PULSETRANsLA'roa Donald D. Grieg, Forest Hills, N. Y., assignor to FederalTeiephone and Badin Corporation, New York, N. EZ., a corporation ofDelaware Application March 19, 1945, Serial No. 583,590

(Cl. Z50-27) 14 Claims.

This invention relates to pulse repeaters and in particular to pulsetranslators or pulse circuits.

An important object of the invention is to produce a pulse repeater inwhich narrow pulses are produced from wide pulses.

A further object is to provide apparatus which may be .designed toproduce a series of pulses of uniform width occurring in rhythm withinput pulses having diierent widths or having variously distorted waveforms, or to produce narrow pulses of uniform width from uniform widerpulses.

A still further object is to control the width of the pulses in theoutput of the repeater by controlling the amplitude of the input pulses,and to produce narrower output pulses as the amplitude is increased.

An ancillary object is to regulate the width of the output pulses byadjusting the slope of Wave form of the input pulse.

A further Objectis to utilize the advantageous operational features of abeam deflector tube in carrying my purposes into eiect.

A specific object is a pulse narrower in which the energy of the inputpulse is used to block the tube after one sharp output pulse has beenformed but before cessation of the input pulse.

Other objects and possible uses of the invention and a betterunderstanding thereof can be derived from the following description ofembodiments of the invention and the accompanying illustrative drawings,in which: y

Fig. 1 represents partly in block diagram and partly in perspective, oneembodiment ofthe invention in which the blocking potential for the beamdeflection tube is derived from a rough differentiator circuit, shown asa simple resistancecapacity network;

Fig. 2 shows a set of curves used in explaining the operation of thecircuit;

Fig. 3 is a View similar to Fig. l showing a second embodiment in whichthe blocking potential for the tube is derived directly from negativeinput pulses;

Fig. 4 represents in block diagram a third ernbcdiment in which aseparate source of grid biasing voltages, such as a multivibrator, istriggered by the incoming pulse;

Fig. 5 shows a set of curves representing the effect of the slope of theinput pulse in controlling the width of the output pulse. A preferredembodiment of my invention comprises electron discharge. apparatus ofthe beam deiiection type in which a tube with an apertured barrierelectrode is used to produce sharp, narrow pulses in its output, of thesame frequency as and automatically synchronized with respect to theinput pulses.

In order to get narrow width pulsesl in the output, a well focused spotbeam is'used, or` a thin beam preferably having a vertical dimension sosmall that thevbeam iits within the edges of the aperture. A thickerbeam requires more time to traverse the aperture at a given speed andproduces an output pulse of slower build up and decay f time.

In the absence of a signal the beam axis of the tube is focused to somepoint above or below the aperture in the barrier, and the normal biaseson the tube may be such that the beam itself is not formed or is ofineiective magnitude until a signal is received. Upon receipt of a pulsethe beam axis is swept rapidly across the aperture, in one direction asthe pulse builds up and in the other direction as the pulse currentfalls to zero'.

The beam itself is, however, suppressed as the beam axis crosses theaperture on the return movement.v v

The energy of the input pulse is utilized both to produce the deflectionof the beam and also to effect its suppression. kFor this latterlpurpose a network is connected to the tube which derives a blockingpotential or both a beam controlling potential and a beam blockingpotential either from the input pulse or under control of the inputpulse, and applies it to the tube over-the proper time interval.Considerable leeway is permissible in the timing; the blocking may beinitiated at any instant after the beam has crossed theaperture thefirst time but it must continue' at least until the beam axis hascrossed the aperture the 'second time in the reverse direction. In oneextreme case and for some purposes of my invention,

the beam may be permitted to passA only While the beam axis istraversing the aperture in the 4o return direction. Or from a broadviewpointof .my invention, the relative time intervals of Vsuppressionand passage of the beam may have any other intermediate value just solong asformation of one output pulse properly related to the input pulseis permitted.

. generally at the time of occurrence of the input pulse series isrepresented at 5 and a pulse amplitude adjusting device at i6. 'Iheseare the fundamental elements of the system and may take various forms,as will be evident as the description proceeds.

Under certain circumstances the delay device may v be eliminated; thepulse amplitude adjuster may also be eliminated where its function isperformed'r by apparatus in the circuit preceding the beam deectiontube. l The term rough dilferentiator is here applied in a broad senseto cover any circuit for deriving from the energy of the input pulse awavegfqrmuheaying the generalnnature of a derivative waye. This will beeeplained more fully in connection with curve B of Fig. 2.

l' The beam deflection tube is shown as `comprising an envelope 9represented in dashlines past the aperture edges,l I9, 20. The Abeamaxis will be restored to its initial position upon cessation of thepulse. As it is restored it will :cross the aperture again in theopposite direction. If

the beam remained eiective at all times an output pulse would beproduced each time the beam.

crossed the aperture.

. Howeveni-the differentiator il 'is :provided for the purpose ofsupplying av negative bias to block and the following electrodestructurefingfthe orderl named: a cathode Il) preferablywoffthethermionic type grounded atII; -agridforbearn 'i control electrode I2, afirst anode i3 foraccelerat` ing the electrons and concentrating thebeam,l a pair of deflection plates Ill` I,andi I5 vvarranged verticallyone above the other with the lower plate I5 grounded at I6, in the usualfashion,'a barrier electrode kI'I having a narrow aperturelfd-arranged-.eentrally in the beam path and hav-l ing upper and loweredges I9 and 2li, respectively, which-intersect a deflection planeperpendicular tof-theplanesof vthe-deiiection*plates I4, Iianda-ollector anode 2l` represented here as a plate n behind the apertureil, on the opposite side of the-1barrierelectrode `I'I from the cathodelil.

'-The' electron-beam tube. may take any one ofV anumber of formsprovided the electrodes are arranged to perform the functionsindicated.-

For example, the` output electrodes may include electrodestructure of vthe secondary' emission type orhoftthe electron.-multiplier type, wheredesired. Accordingly'theshowing ofthe output electrode-'as a simplecollector anode behind the barrierelectrode 'I1 `is not to be tak-en aslimi/ting the invent-ion to the use cfa tube havingthis particular,structure. Likewise, otherY types Yof beam `ldejlecting means suchasthoseusing magneticelds could be used where possible. `Only so much oftheVV tube connections are ilmenate@ .as is neeessary to explain theOperation ofgthecircuit. v Thus, the connection of the barrierhelectroleHand collector anode; 2l to a e vofI positive potential, marked fB-l-Lis indicated! but the complete circuits are not shown. Zlll'e rough.Vderentiator 4 may comprise .a

simple resistancecapacity network as illustrated ingls. .1.coinn0sed of.C'ondenserland resistor .13,tnel-1attengrounded..at 24- The Commonterminal pf condenserZZand resistor 23 is connetted to. thesrid. l2 oftube?. andthe opposite .terminal of the condenser 1 '2l iswnnetedto .the

pulseinpllt circuit.

Theypulse vinput Lis illustrateduan connected i tatnetubeQimuitsjinrenehe resistance. capacity icounlineknown vin itself. and.@gmx-arising .con-

vdenserg., andua resistance 25 grounded at 27. At .,a-,point128)gieyoridy the coupling to line, ythe `ifi-1.1156inputciricnit,isl@renhfd.,` to supply input .nillsestotwo nathanne by ,Way ofthe delaydence 5 and, pulse amplitude. tdiustinf-i` means .6to thev ,deflectionAPlates M, yI5 ,of-,tube 3 and the other by way of thediferentiatorncircuit 4.to.grid. l2of tube 3.

'Iheperation. ofwthe circuit'of 1 isas n follows:

. The beam axisof the tube is normally focused toapoint above vore belowthenaperture yIll depending on the polarity Qfihedectinspulse- Tnis maybeaccornplished by. applying asuitable steady .PQtential to -thedcectgrl. plates f lQm a directcurrent source A[6a. Whenltlhe inputpulse applies-1apetential t0 deectionpiate M pf. the tube, the pulsewill cause the beam to sweep pulses.

the beam, preventing the formation of an'output f pulse upon return`movement ofthe beam axis to its i normal .position after deflection-`The A diiferentiator also' serves the additional purpose ofsupplyingapositiveibias to bring the beam upv to full strength by thetime it crosses the aperture upon its Vdeilection away from normalposition.

The leading edge of vthe input pulse produces the positive biaspotential and the trailing edge produces the negative bias potential, atthe .terminal of the diierentiator circuit connected to the grid.Bothbiasing potentials must belproperly :timed to be effectiveas thebeam axis crosses thefaper# ture, the positive bias as the beamv movesfrom its initial position and the negative biasffasnit returns to itsinitial position. The positive bias must also be so .adjusted as'tomagnitude as to assure Ypassage of van output kpulse of. proper. n

amplitude.

.The width of the output pulse is dependent on the width of "theaperture measured in the line of deflection yof the beam, Yon thelcorresponding dimension of the beamand on the yspeed at which' the beamcrosses the aperture. InV order; thereforerto get narrow output pulsesthe dimensions andiarrangementrof the aperture and-the size of the beaminthe plane of the apertureimustbe properly chosen. In addition, thetube, properly constructed and properly biased, must also `be sooperated as to rcontrol the speed atwhich the` beam isdeected across theaperture, Twofactors'may control the speed of'traverse, In' the case` ofinput pulses having a leading edge which rishsubstantially vertical, thebeameforming voltage is appliedsubstantially instantaneouslyand thespeed Ais greaterasjthe appliedl voltage in-y creases. AAc'cordingly,Vamplifying the weave willH increase thespeed andnarrow the output pulse.If the input Apulse is sinusoidal in form, amplication will alsoincrease the speed of deflection. In this case Athe wave of greater peakamplitude has a higher rate of change of voltage, that is, a greaterslope.' The slope of Athewave will affect I the sp`eeda wave of vgreater slope giving a narrower output pulse.

It is obvious that the fundamental reason whyk the circuit can be usedas a pulse narrower` is Vthe fact that output pulse current, flows onlyduring a partr of the vtii'nej that an inputv pulse is nowing. By`suppressing the'` second of each pair of pulses thatwould be producedifv the diiferenv tiatorl were vnot present thefdevice serves to re peata pulse rseries withouty changing the frequencyA of the pulses or theirrelative spacing provided there is vno appreciable difference in the.

slopes Aof the leading edges of the individual input these circumstancesvmay be repeated as narrow vpulses oficonstant width and unchangedfrequency. The narrowing operation produces an output pulse which'ycorresponds in time position` with the leading edge of the widepulses;r Ac-` cordingly1 rpulses ofthe Xed leading edge type may berepeated as luniformly spaced, narrow pulses of desired amplitude. y

In order to explain in greater detail the action Thus, Ypulses ofYvariable width under of the delay device 5 in timing the circuit,reference is made to Fig 2. In this iigure, curve A represents pulseinput, curve B the output of the diierentiator circuit applied to gridI2, and curve C the output of the beam deiiection tube. On curve A thefull line wave form 30 represents the pulse applied to grid I2 and thedotted line wave form 3l the input pulse before it is delayed. A pair ofparallel horizontal dash lines 32, 33 intersecting wave forms 3B and 3lillustrates the clipper action of the aperture land is here designatedthe equivalent gate of the tube. The heights of these lines above thezero axis represent the sweep voltages at which the beam enters andleaves the aperture. For the leading edge of the input pulse the lowerline 32 represents the sweep voltage as the beam enters the aperture andthe higher line 33 the voltage at which the beam leaves the aperture.

A pair of vertical lines 34 and 35 between curves A and B are drawnthrough the points of intersection of line 32 with wave forms 33 and 3l.The space t between lines 34 and 35 represents the time delay introducedbetween the input pulse applied to the circuit of the differentiator 4and the input pulse applied to the deilection plates I4, I5. This delayis produced artioially by delay device 5, to the extent that it is notinherent in the electrical'constants of the circuits. Its purpose is topermit the grid I2 to acquire a sufciently positive bias from the Outputvoltage of differentiator 4 so that a beam of proper electron densitywill flow as the beam axis is swept across aperture I8. Preferably theconstants of the differentiator are adjusted so that the grid receivesits peak positive bias while the beam passes the aperture- The timing ofthe delayed input pulse with respect to the derived wave form isrepresented on curve B of Fig. 2. Curve B shows in an approximate formthe wave form of the bias supplied by a rough differentiator circuit ofthe type illustrated in Fig. 1. This wave form 36 supplies a positivebias potential as the beam axis sweeps across the aperture from itsinitial posi tion, followed by a negative bias potential. If a beamnormally flows in the tube this positive bias serves to control itsdensity. If the beam is normally blocked this positive bias counteractsthe normal blocking bias and permits formation of the beam as well ascontrolling its density.

Wave form 36 may be analyzed as comprising four portions, 31-38-39-43,of which 3T and 39 play significant roles. The part 3l represents asharp rise in positive voltage produced at the leading edge of the inputpulse, and portion 39 a corresponding peak of negative voltage producedat the trailing edge of the pulse. The wave form 36 is shown smoothedout somewhat by the constants of the circuit. The intermediate portion38 represents the decreasing potential drop across resistance 23 ascondenser 22 approaches full charge. The shape of the part 38 is notimportant since it represents the bias on grid I2 for the intervalbetween the two aperture crossings of the beam axis. Part 40 representsthe time interval during which the condenser 22 discharges and theblocking bias on grid I2 disappears.

The curve C represents the narrow output pulses produced in the outputcircuit of tube 3. As shown a single pulse 49 is produced for each inputpulse, the spacing between the two narrow output pulses corresponding tothe spacing between the leading edges of successive input pulses.

The shape and amplitude will depend on various factors previouslydiscussed. Ii :a sharp output pulse is desired care should be taken thatthe beam is properly focused. If the beam is wide` .the output pulsedepends lon the speed of deection .and may be regulated by adjusting theamplitude or slope of the leading edge of the input pulse; the shape ofthe output pulse depends on the characteristics of the beam and may becontrolled by proper choice of tube and tube circuit constants.

Referring to Fig. 3, the diferentiator circuit of Fig. l is herereplaced by a delay circuit 4I. The circuit of Fig. 3 is used where theinput supplies pulses of negative polarity which may be applied to thegrid in delayed form to suppress the beam after it has been deflectedonce across the aperture. In this case the beam is normally eiiectivebut is focused to one side of the aperture, the side opposite that whichwould be used for positive input pulses. Assuming that it is focusedabove the aperture I8 and that a negative input pulse applies a negativepotential to deiiection plate I4, then the delay introduced by devicefil must be such as to apply the delayed negative pulse to the gridbefore the beam axis .reaches the aperture on its return sweep back tonormal position. The operation is otherwise similar to that of Fig. l.The shunt resistance 613A allows the charge on the shunt capacitance 43to leak off and thus make the device receptive for the next pulse.

Although the delay device 4I is represented-as comprising a seriesresistance l2 and shunt capacity d3, and shunt resistance @3A thisshowing is merely representative and any suitable delay circuit may beused.

The system of Fig. 4 differs from those of Figs. l and 3 in that aseparate source 44 of blocking potentials is provided. rThis source istriggered by each incoming pulse to supply blocking potentials or bothbeam controlling and blocking v potential at the proper time, inaccordance with the principles of the invention as already eX- plained.A multi-vibrator may provide the blocking potentials, but any source issuitable which may be made to supply a blocking voltage at the propertime. Ii the source provides only negative pulses the tube must beoperated with the beam normally effective, but if the source providesboth positive and negative pulses as does the diierentiator circuit ofFig. l, then the tube may be operated with the beam normallyineffective, a positive pulse being applied to bring the beam to fulldensity as it is deflected across the aperture in the barrier electrode.Since the operation of the circuit of Fig. 5 is very similar to that ofthe circuit of Fig. l a detailed description of its operation isbelieved to be unnecessary.

Fig. 5 shows two curves D and E. On curve D a flat-topped input pulse 45and a sinusoidal input pulse it oi approximately the same maximumamplitude are shown, a pair of spaced horizontal lines representing theclipper action of the aperture in the barrier electrode of tube 3. CurveE represents in full lines the pulses 49 and 5I which are actuallyproduced at the leading edges of the rectangular and sinusoidal pulses,respecl tively. 'I'hedottedvl linesf5| and 52 represent the pulses 'iproduced at the trailing edges of pulses 45 and 46 if no blocking ofthetube occurs. The pulses l and 52 do not .appear in the output circuitsince provision is made for suppressing them by theI blocking bias fromthe dierentiator of Fig. 1 or an equivalent means. It will be noted thatthe rectangular pulse i5 with the steeper slope at the aperture gatewill produce narroWer pulses 49, 5l spaced farther apart than Will asinusoidal pulse of the same peak amplitude, since the latter has lessslope at the aperture gate l It will be apparent that in the embodimentsof the invention illustrated, the wave form,` the amplitude and theWidth of the output pulses do not necessarily correspond with those ofthe input pulses but may be controlled by them to the extent indicated`However, the frequency of the input and output pulses is the same due tothe fact that the'second of each pair of pulses produced by the inputpulse is suppressed. Moreover, with this invention a series of uniformpulses may be shifted as a whole in time position as they are narrowed,the relative time positions of the individual pulses remainingunchanged.

Various possible changes in the circuits will be obvious to thoseskilled in the art. For example, the function of varying the amplitudeor of adjusting the slope of the input pulse may be performed inapparatus preceding the deiiection tube circuits rather than in the pathbetween the pulse input I and the deflection plate I4. The adjustmentmay be made manually or automatically by a variable amplifier placed ineither position. It is also possible to place the variable amplifier orpulse amplitude adjuster either before or :after the delay device 5.

1. A system for repeating pulses comprising a beam deilection tube withinput and output circuits therefor, and having an output electrode beamcollecting means and a barrier electrode provided With an aperturepositioned across `the beam deflection plane, means for generating anddirecting an electron beam toward theoutput electrode, means to causedeection of said beam across said aperture in a xed time relation withrespect to the occurrence of each input pulse, means responsive to adirect current voltage component of a given polarity and derived fromeach input pulse to control the intensity of the beam to'prcduce in saidoutput circuit a corresponding pulse, and means controlled by a directcurrent voltage component of the opposite polarity and derived from theinput pulse and synchronized With the input pulse topreventproductionof.- an additional pulse inthe output circuit.

2. A system according to claim 1 wherein the input circuit has means forderiving from the input pulse a control voltage and .for applying thesameA in synchrcnism With the deiiection voltage to suppress the beamand thereby prevent the production of additional pulses in responsetosaid input pulse,

3. A pulse repeating circuit comprising an in' putv circuit for pulses,a beam deflection tube comprising beam forming, beam controlling, beamdeflecting and beam collecting electrode structure and a barrierelectrode in the path of the beam having a pair of edges across whichthe beam moves successively upon deilection, means for deriving,` fromthe input pulses a variable voltage wave having beam passing and beamblocking values,.means for: applying the input pulse tothe -mc'vessuccessively upon deflection, means for deriving from the input pulsesand applying to the beam controlling structure a varying'voltage Wavehaving beam passing and beam blocking values Vand means for delaying theinput pulses and applying them to the deflecting structure.

5. A pulse repeating circuit comprisingY an input circuit for pulses, abeam deflection tube vincluding beam forming, beam controlling, beamdeflecting and beam collecting electrode structure and a barrierelectrode in the path of the beam A provided with an aperture having apair ofedges across which the beam moves successively upon deflection,means for deriving from `the input pulses a variable voltage Wave havingbeam blocking values, means for amplifying the input pulses and applyingthem in amplied form to the def' rived` Wave are effective beforenesting structure so as to regulate the speed at which the beam isdeflected across the aperture in the barrier electrode, means forapplying the derived Wave to the beam vcontrolling electrode structureso timed that blocking values of the decessation of an input pulse. Y

5. A pulserepeating circuit comprising an input circuit for pulses,abeam deflection tube including beam forming, beam controlling, beam.

deecting and beam collecting electrod-e structure and a barrierelectrode in the path of the beam having a pair of edges across whichthe beam moves successively upon deflection, means for deriving from theinput pulses a varying voltage Wave having beam permitting and beamVblocking values, means for regulating the amplitude of the input 'pulsesand for applying them to the deiecting structure, means for applying thederived Wave to the beam controlling structure'so that a beam permittingpotential is effective upon receipt of an input pulse and a beamblocking potential becomes effective prior to cessation of the inputpulse. f

.'7. A pulse repeating circuit including an input circuit for pulses, abeam deflection tubehaving beam forming, beam controlling,'beamdelecting Y and beam collecting means and a barrier electrede in thepath of the beam having a pair of edges intersecting a deiiection planeof said beam, means for applying the input pulses to the deiiectingelectrode means, means for deriving under control of the received inputpulse a varying voltage wave having beam blocking values,A said lastmentioned' means being self-synchronizing With respect to the pulseapplied to the deecting means to initiate beam blocking values duringintervals intermediate the leading and trailing edges of the saidapplied pulse, and meansfor applyingr said voltages having beam blockingvalues to said beam controlling means to cause blocking of the beam.

8. A pulse width converter comprising an electron discharge device ofthe beam deection type having beam forming, beam controlling, 'beamcleecting andbearn collectingmeans `and Han,

apertured barrier electrode in the path of the beam, means for initiallyfocusing the beam to a point to one side of the aperture, a source ofinput pulses for the beam deilecting means for deilecting the beam alonga line passing through the aperture, means for controlling the speed oftraverse of the beam across the aperture, means under control of aninput pulse for preventing passage of the beam across the aperture inthe opposite direction, and a pulse output circuit connected to the beamcollecting means.

9. A pulse width converter comprising an electron discharge device ofthe beam deflection type having beam forming, beam controlling, beamdeiiecting and beam collecting means and an apertured barrier electrodein the path of the beam, means for initially biasing the beam to a pointto one side of the aperture edge, a source oi input pulses for the beamdeflecting means for deecting the beam back and forth across theaperture in response to each incoming pulse, and means under control ofthe input pulse for suppressing the beam on movement of the beam in onedirection across the aperture,

10. A pulse Width converter comprising an electron discharge device ofthe beam deflection type including beam forming, beam controlling, beamdefiecting and beam collecting electrode structure and lan aperturedelectrode in the path of the beam, means for initially focusing the beamto a point at the side of said aperture, a source of input pulses forthe beam deilecting electrode structure for deecting the beam along aline passing through the aperture, means for controlling the amplitudeand time phase of the pulses applied from said source to the beamcontrolling electrode structure, means for modifying the input pulsesand applying them in modified form to the controlling electrodestructure so as to produce therefrom blocking potentials for the beam,and a pulse output circuit connected to said beam collecting electrodestructure for receiving the pulses converted by said electron dischargedevice.

11. A pulse width converter comprising an electron discharge device ofthe beam deflection type having beam forming, beam controlling, beamdeecting and beam collecting electrode structure and an aperturedbarrier electrode in the path of the beam, means for normally focusingthe beam to a point on the barrier electrode to one side of saidaperture, a source of input pulses for the beam deecting electrodestructure for deecting the beam back and forth across the aperture,means for modifying the input pulses and applying them in modified formto the controlling electrode structure so as to produce therefromdelayed pulses as blocking potentials for the bearn, and a pulse -outputcircuit connected to said beam collecting electrode structure.

12. A pulse Width converter comprising an electron discharge ydevice ofthe beam deection type comprising beam forming, beam controlling, beamdeflecting and beam collecting electrode structure and an aperturedbarrier electrode in the path of the beam, means for normally focusingthe beam to a point on the barrier electrode to one side of saidaperture, -a source of negative pulses connected to 'said beam deectingstructure, means for delaying the received negative pulses and applyingthem to sai-d beam controlling structure in delayed form to provide'delayed blocking potentials for said electron discharge device, and apulse output circuit connected to said electron discharge device.

13. The method of operating a beam `deflecting tube of the aperturedbarrier electrode type which comprises applying defiecting voltages tosaid tube under control of input pulses in such manner as to cause thebeam of the tube to move from a position at one side of the aperture toa position at the other side, the movement being in one direction sasthe pulse builds up and in the opposite direction as the pulse decays,and While maintaining beam passage during movement in one of thedirections suppressing the beam during its movement in the otherdirection.

14. The method of operating a beam deflection tube of the barrierelectrode type which comprises focusing the beam to a point on thebarrier electrode to one side of the aperture, deflecting the beam at acontrolled speed from its position of initial focus to a position at theopposite side of the aperture by deflecting voltages derived fromincoming pulses and preventing movement of the beam across the aperturewhen the deflecting voltage restores the beam to its position of initialbias, by blocking the beam with a `delayed blocking potential derivedfrom the deiiecting voltage.

DONALD D. GRIEG.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES lPATENTS Number Name Date 2,011,920 Terry Aug. 20J 19352,096,653 Soller Oct. 19, 1937 2,191,185 WoliT Feb. 20, 1940 2,225,330Cage Dec. 17, 1940 2,262,407 Rath, Jr. Nov. 11, 1941v 2,365,476 Knoop etal. Dec. 19, 1944

